1. Field of the Invention
The present invention relates to a zoom lens, an imaging device, and a personal digital assistant.
2. Related Art
Accompanied with the wide spread of an imaging device represented by a digital camera, the general trend seeks for further improvement of a photographed quality and further downsizing of the device body, and it seeks for higher performance as well as further downsizing of the zoom lens used as a photographing lens.
In view of further downsizing of the zoom lens, first, the total lens length in use (distance from the lens surface on the most object side to the image surface) has to be shortened, and it is important to shorten the thickness of each lens group and to control the total length when the zoom lens is put in storage.
In view of higher performance, it is preferable to secure a resolving power corresponding to an imaging element of at least 5 to 8 million pixels, throughout the whole zooming range.
Further, the wide angle of view of the zoom lens is strongly requested, and it is preferable that the half angle of view at the wide-angle end is over 38 degrees “corresponding to the focal length 28 mm in terms of the 35 mm silver halide camera (the so-called Leica type)”. The magnification ratio is also desired to be as high as possible; provided that the zoom lens has the magnification ratio corresponding to the focal length 28 to 200 mm in terms of the 35 mm silver halide camera (about the multiplication 7.1), it is considered as possible to deal with almost all of general photographing. Further, a zoom lens with the magnification ratio corresponding to the focal length 28 to 300 mm (about the multiplication 10.7) will cover a still wider photographing zone.
On the other hand, a chromatic aberration is likely to develop accompanied with pursuing a higher magnification ratio and a longer focal length; and it is well-known that the use of a “lens having an anomalous dispersiveness” is effective for the correction of the chromatic aberration.
As a zoom lens suitable for a higher magnification ratio, there is an already-known configuration such that a first lens group has a positive focal length, a second lens group has a negative focal length, a third lens group has a positive focal length, and a fourth lens group has a positive focal length, in order from the object side, and when changing magnification from the wide-angle end toward the telephoto end, a spacing between the first lens group and the second lens group increases, a spacing between the second lens group and the third lens group decreases, and a spacing between the third lens group and the fourth lens group varies. A zoom lens having such a configuration and using an anomalous dispersiveness is disclosed in the following patent document 1 through the document 3 and so forth.
The zoom lens disclosed in the patent document 1 has 25 degrees of the half angle of view at the wide-angle end in the embodiment disclosed; this cannot answer the demands of recent users in terms of seeking a wider angle. The zoom lens disclosed in the patent document 2 has about 29 to 32 degrees of the half angle of view at the wide-angle end, which cannot be considered as sufficient in terms of a wider angle. The zoom lens disclosed in the patent document 3 has about 37 degrees of the half angle of view at the wide-angle end, which can be recognized as an ample wider angle; however, to achieve this required as many as 14 pieces of lenses in total, and there is conceivably a room for improvement in terms of downsizing (reducing the total length in storage) and reducing the cost.
Patent Document 1: Japanese Patent Laid-Open No.H08-248317
Patent Document 2: Japanese Patent Laid-Open No.2001-194590
Patent Document 3: Japanese Patent Laid-Open No.2004-333768